A method is described for measuring amino acid uptake by the syncytiotrophoblast of the human placenta in vitro using taurine as an example. Small fragments of placental villous tissue (2-3 mm3) are tied to a comb that enables them to be moved in concert between a series of incubation and wash buffers. The fragments are incubated in control or Na(+)-free Tyrode's buffer containing 3H taurine for timed intervals. Following incubation, the tissue is washed, lysed in water to release the accumulated isotope, and finally denatured in NaOH to determine protein. The water lysate is counted for radioactivity and the Na(+)-dependent component of 3H taurine uptake calculated as the difference between uptake in control and Na(+)-free conditions. Because the taurine transport protein (system beta) is Na(+)-dependent, the Na(+)-dependent component of 3H taurine uptake gives a measure of system beta activity in the microvillous membrane of the syncytiotrophoblast. The method described here in relation to taurine can be applied to other Na(+)-dependent amino acid transport systems. An advantage of the fragment model is that syncytiotrophoblast metabolism and signaling are retained. A disadvantage is that fragments contain many cell types and carrier-mediated amino acid uptake may be into several compartments including syncytiotrophoblast.